What is a solar eclipse?

On Aug. 21 we will see the first solar eclipse in the U.S. since 1979. Our graphic explains exactly what one is, shows its path and some how-to viewing tips. By Ramon Padilla
Karl Gelles, Dann Miller, Walbert Castillo, Janet Loehrke and Sara Wise, USA TODAY NETWORK

The moon appears to cover the sun during an annular eclipse of the sun on May 20, 2012 as seen from Chaco Culture National Historical Park in Nageezi, Ariz.(Photo: Stan Honda, AFP/Getty Images)

Unless you've been living under a rock, you've probably heard about the solar eclipse on Aug. 21. So what is a solar eclipse, anyway?

A total solar eclipse occurs when the moon passes between the sun and the Earth. The moon appears to completely cover the disk of the sun. By blocking the sun's light, the moon casts a shadow that turns day into an eerie twilight here on Earth.

Eclipses occur because of the special coincidence of the moon and the sun being the same size to our eyes. The sun is 400 times wider than the moon, but it is also 400 times farther away, so they appear to be the same size in the sky.

Great American Eclipse(Photo: USA TODAY)

If you live in the "path of totality" — the 67-mile-wide area in which the moon's shadow sweeps across the Earth's surface — you'll see a total solar eclipse, weather permitting.

That path will cross parts of 12 states, all the way from Oregon to South Carolina.

If you're not in that path and skies are clear, you're still guaranteed to see a partial eclipse, in which only part of the sun will be blocked by the moon.

A partial eclipse will be visible in all of North America, parts of South America, western Europe and Africa. While a partial eclipse is still cool, you won’t notice your surroundings getting dark.

When the moon does eclipse the sun, it produces two types of shadows on Earth: The umbral and penumbral shadows. The umbral shadow is the relatively small point on Earth where you'll see a total eclipse.

The penumbral shadow is the much larger area on Earth, where a partial eclipse will be visible.

Historic Charleston, S.C., has rolled out an array of packages for the eclipse, from hotel rooftop viewing with a College of Charleston astronomy instructor, to a special blues-and-barbecue harbor cruise on a paddlewheeler. Explore Charleston

Parts of Great Smoky Mountains National Park will experience more than two minutes of totality. The park has organized three viewing areas, including Cades Cove, Oconaluftee and Clingmans Dome, which will be by ticket only. Akos Kokai

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In addition to total and partial eclipses, sometimes annular eclipses occur. An annular eclipse happens when the moon covers the sun's center, leaving the sun's visible outer edge to form a “ring of fire” or "annulus" around the moon.